1. Field of the Invention
The present invention concerns a method for manufacturing a polyimide thin film which can be used as an orientation film for liquid crystals, substrates for forming organic compound crystal thin films, etc., and an apparatus for the same.
2. Description of the Prior Art
Recently, liquid crystal displays have been increasingly utilized and, since the display is realized by controlling the orientation of the liquid crystal molecules on the surface of a substrate, or orientation film, improvement in the performance of the orientation film for liquid crystals has become important.
Orientation of liquid crystal molecules relative to a substrate can be generally classified into three types: (1) in parallel with, (2) vertical to or (3) slanted to the surface of a substrate, and various kinds of display have been actually attained by the combination of (1), (2) and (3) in a practical point of view. These situations generally depend on a physicochemical effect between the surface of the substrate and liquid crystal molecules, or the direction and magnitude of various elastic forces accompanying the orientation of the liquid crystal molecules on the surface of the substrate, etc. The physicochemical effect is formed due to Van der Waals force between a coating film or surface treated material on the surface of the substrate and liquid crystal molecules, polarization ratio, chemical bonding force, etc. For instance, it has been known that liquid crystal molecules can easily be oriented in a certain direction relative to the surface of the substrate by applying a physical or configurational treatment to the surface of the substrate, which has been utilized generally. This technique is attributable to a low elastic energy caused by the distortion of the liquid crystal molecules when they are oriented in the direction of concavo-convex stripe traces formed on the surface of a substrate. Further, considering the application of liquid crystals to a display device, the elastic property of the liquid crystals per se is an important factor in addition to the factor on the surface of the substrate.
Then, referring to a method of manufacturing an orientation film (substrate) for liquid crystal molecules utilizing such various kinds of forces exerted on each other, a method of orienting liquid crystals by use of a chemically modified surface of an inorganic substrate such as glass thereby providing a physico-chemical force can include (1) a method of obtaining an orientation film by coating a surface-active-agent such as a phospholipid material which is an amphiphatic material (e.g., lecithin) or a silane compound or a synthetic polymeric material such as polyamide, polytetrafluoroethylene, etc., on a substrate for liquid crystals, (2) a method of obtaining an orientation film by depositing an inorganic chemical absorbent of higher heat resistance and relatively great endurance such as a carboxylate-chromium complex on a substrate for liquid crystals, etc. On the other hand, a rubbing method is known which includes rubbing a glass substrate with fresh cloth or jewel abrasive and utilizing the fact that the orientation of liquid crystal molecules is sensitive to the concave-convex surface of the substrate. Another method of mechanically inprinting concavo-convex stripe traces is known using a replica and thereby obtaining an orientation substrate (orientation film). Further, liquid crystal molecules may also be oriented by merely forming a geometrical pattern and, for example, there has been reported a method of forming an orientation film by utilizing a sort of uneven vapor deposition formed by oblique vapor deposition of an inorganic element (Au) or an inorganic compound (SiO.sub.2). In the oblique vapor deposition method, since the mean free path of atoms or molecules in vacuum is generally as long as from several tens of centimeters to several meters, uneven vapor deposition is characteristically formed only in one direction. It has been known from an example of research report that the mean height of these geometrical patterns of less than 100 Angstroms can sufficiently orient the liquid crystals, and the orientation film formed by the oblique vapor deposition method avoids concerns about whether an electroconductor is used or not. There has also been a method of obtaining an orientation film by forming concavoconvex stripe traces by the combination of a physical method subsequent to the chemical processing such as applying oblique etching by ion beams to the surface coated with a surface active agent.
In orientation films for liquid crystals in which various surface active materials are coated on a transparent glass substrate, there have been problems in view of practical use since the materials used so far are poor in durability and, with regard to a glass substrate attached to a transparent electrode or display element, a lot of metals, polymeric compounds, non-metal elements, etc. are laminated on the substrate which worsens the durability and cause unevenness in the adhesion. Further an attempt has been made to determine the technical factors necessary for the production of liquid crystal orientation films prepared by mechanically printing the concavo-convex stripe traces since the manufacture is simple in principle. However, since this is mechanical rubbing, there are several problems such as unevenness upon printing, poor reproducibility, intrusion of impurities, etc. These problems have become more severe as higher quality and reliability have been demanded for the liquid crystal orientation film.
For overcoming the foregoing problems, it has been developed in recent years a method of forming an aromatic polyimide series thin film of excellent heat resistance, chemical stability, dielectric property, etc. on a glass substrate attached to a transparent electrode or on a glass substrate attached to various display elements and, thereafter, applying convavoconvex stripe traces mechanically, thereby forming an orientation film. The method is excellent over other various methods in that substrates made of various materials can be chemically modified uniformly and then subsequent formation of concavo-convex stripe traces can be attained at a relatively good reproducibility, and the durability of the liquid crystal orientation film can progress remarkably by the development of this method. However, the geometrical shape of the liquid crystal orientation film has become complicated more and more along with increase in the integrated circuit density and the performance of devices, and a technique capable of forming a polyimide thin film with no unevenness due to a uniform thickness and with good adhesion irrespective of positions has become an important factor for attaining the demanded performance of liquid crystal display devices. For instance, in a polyimide thin film formed by a coating method, drying and heat curing a polyamic acid solution on a substrate for liquid crystals, the thickness of the film at protruded portions such as electrodes or display devices is reduced, the unevenness in the coating thickness upon forming the liquid crystal orientation film causes unevenness in the formation of concavoconvex stripe traces, separation of the orientation film, etc. which often leads to failed liquid crystal orientation, or electrical leakage to result in fatal troubles, which can not be overlooked.